How to install QGIS 3 using OSGeo4W in Windows OS

The Semi-Automatic Classification Plugin (SCP) version 6 (codename Greenbelt) will be released very soon.
This new version is compatible with QGIS 3 only; therefore you need to install the QGIS development version until QGIS 3 is officially released.

This post is a brief guide about how to install QGIS 3 in Windows OS using the OSGeo4W installer.